- December 15, 2011, Mark Miller
Why would you coat with a roller instead of a knife; a rod instead of a slot die? There are many methods for fluid coating and just as many techniques for utilizing the equipment. Outside of space constraints, original equipment manufacturers (OEMs) have made it possible to slide in almost any fluid coating equipment as a replacement for existing equipment. So what do you have to choose from? Check out the table below.
|Roll Application||Pre-Metered Application|
|Direct gravure||Slot die|
|Reverse gravure||Rotary rod|
Coating methods fall into two major areas: roll application and pre-metered application. Pre-metered application typically requires a slot die where a pump and line speed of a substrate determine the fluid coat weight. Roll application requires that the fluid flow between two rotating rolls that dictate the fluid coat weight and the uniformity of the coated fluid across the substrate. The table presented above does not cover every possible coating method or variation of those presented. Many coating facilities have developed methods and techniques that are unique to an individual product.
With so many options available, a good place to start is with product specification. Let’s take, for example, an optically clear coating that needs to have reduced debris from handling and coating. This low level of contamination leans the decision toward pre-metered coating contained within a slot die. If the coating does not require contamination control, but instead requires quick change-over from one product to another, a self-metered arrangement with a comma coater may be sufficient.
Another consideration should be the scale of the fluid coating. There is a distinction between lab, pilot, and production scale that plays into the choice of methods. Many times a Mayer rod may be used in the lab to allow for multiple handspreads to be completed in a short time. This is great if you are scaling to a production Mayer rod manufacturing facility, but there are many process factors that will not scale properly if the production facility runs a gravure roll instead. If possible, the method should be based on the physical parameters of the product (required cross-web uniformity, volume of fluid to be coated, solution characteristics, number of coating layers, and substrate type).
Don’t forget to keep in mind expertise! Even if the product requires a specific type of coating method, the fluid coating operators have developed an understanding and skill with the current coating method used in your facility. Training on a new coating method will require some downtime, and this needs to be factored into the return on investment in the newly installed equipment.
The bottom line, however, is always the bottom line. The cost of one fluid coating method can vary substantially with the cost of another fluid coating method. Do not look simply at the sticker price however! Make sure to keep in mind the requirements of changeover, replacement parts, precision rolls, and process limitations (line speed and rheological design of the fluid). The decision of which fluid coating method to utilize is multifaceted and requires some thought. Good luck and happy coating!
Roll-to-roll coating industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slot die coating experience and troubleshooting. Contact him at 612-605-6019; firstname.lastname@example.org; www.coatingtechservice.com.